Conventionally, retroreflective materials that retroreflect incident light are widely used for indications such as traffic signs, or for identification of marine accident equipment, and particularly for enhancing visibility at nighttime. From the viewpoint of ensuring the safety of people who work at night, such retroreflective materials are also widely used as safety clothing for policemen, firefighters, workers involved in civil engineering and construction, and the like, in safety clothes, safety vests, sashes, arm bands, life vests, and the like. Further, in recent years, along with a growing consciousness of the safety of life, or the diversification of decorativeness, such retroreflective materials are also used in apparel such as windbreakers, sweat suits, T-shirts, sports shoes, and swimming suits as measures for preventing traffic accidents at nighttime, or used in bags, suitcases, and the like for decorative purposes.
A typical retroreflective material has a structure in which transparent microspheres are provided on a reflective layer, whereby light incident through the transparent microspheres is reflected at the reflective layer, and light is emitted through the transparent microspheres, so that light is retroreflected. In the retroreflective material with such a structure, a transparent resin layer may be provided between the reflective layer and the transparent microspheres to adjust the reflective luminance or the color tone of the reflected light. Conventional retroreflective materials are broadly classified into the three types, i.e., an open type, a closed type, and an encapsulated type, depending on the manner in which the transparent microspheres are embedded. In an open-type retroreflective material, a portion of the transparent microspheres are exposed in the air (see, for example, Patent Literature 1). In a closed-type retroreflective material, surfaces of the transparent microspheres (surfaces positioned opposite to the surfaces facing the reflective layer) are covered with a resin layer (see, for example, Patent Literature 2). In an encapsulated-type retroreflective material, there is space over the surfaces of the transparent microspheres (surfaces positioned opposite to the surfaces facing the reflective layer), and a resin layer is present over that space (see, for example, Patent Literature 3). Among these types, open-type retroreflective materials find wide application in the field of clothing, because they have high reflective luminance as well as flexibility.
In recent years, in response to consumer needs such as the diversification of decorativeness and a growing liking for high-grade products, there is a demand for the development of a retroreflective material that can display original colors. To meet such consumer needs, some retroreflective materials have previously been reported which not only exhibit a monochromatic color tone, but also exhibit a plurality of colors depending on the incident angle of incident light. Patent Literature 4, for example, discloses a retroreflective material including a single-layer interference layer (reflective layer) composed of a specific metal compound directly deposited on transparent microspheres, wherein gradations within the range of 100 to 600 nm are imparted to the layer thickness of the interference layer, which allows a plurality of color tones to be produced depending on the incident angle of incident light. The retroreflective material disclosed in Patent Literature 4, however, has a drawback in that with respect to incident light with the same incident angle, the hue of the reflected light is uneven depending on the region of the retroreflective material, and color unevenness readily occurs. Thus, this retroreflective material cannot thoroughly satisfy the consumer needs that have grown recently. Further, in the retroreflective material described in Patent Literature 4, the single-layer interference layer (reflective layer) composed of a specific metal compound is directly deposited on the transparent microspheres; in this structure, however, the thickness of the interference layer needs to be increased to approximately 400 nm to obtain more multicolored interference colors, leading to a very high vapor deposition cost.